The present invention relates to an I-axis detecting circuit which is used in a color demodulating circuit provided in a TV receiver of an NTSC system.
Two axis detecting circuits are provided in a color demodulating circuit of the NTSC-system TV receiver. To efficiently transmit a color signal, the NTSC system uses the color resolution of human eyes, which is superior in a cyan system belonging to an orange system (corresponding to the I-axis shown in FIG. 6) but inferior in a green and magenta system (corresponding to the Q-axis). This means that the NTSC broadcasting system is designed to make the transmission band in the I-axis direction wider and transmission in the Q-axis direction narrower for the purpose of efficiently transmitting a color signal.
Further, when transmitting a color signal in the NTSC system, the color signal is modulated in a balanced manner and overlapped with a luminance signal. At the TV receiver, the color signal is separated from the overlapped signal. Then, the I axis and Q axis are detected in the color signals and the colors are demodulated along the I axis and the Q axis. This is referred to as two-axes color demodulation.
One prior art method of the two-axes color demodulation is described in "NHK TV Technical Text" (first volume, edited by Nippon Housou Kyoukai). In the two-axes color demodulation, the same principle holds true of the I-axis and the Q-axis color demodulation. Hence, only the I-axis detecting circuit will be described later.
Herein, the description will be directed to one example of the conventional I-axis detecting circuit. The conventional circuit is shown in a block diagram of FIG. 5.
In FIG. 5, 1 is an input terminal at which a chrominance signal S.sub.1 is applied, 2 is an input terminal at which a color burst signal S.sub.2 is applied. 3 is a band amplifying circuit, 4 is an automatic color control (ACC) circuit, 5 is a circuit for amplifying a color burst signal, 6 is a circuit for generating a 3.58-MHz signal, 7 is an I-axis demodulating circuit, 8 is a phase shift circuit, and 9 is an output terminal at which an I-axis detected output appears.
The function of the I-axis detecting circuit arranged as above will be described below.
The chrominance signal S.sub.1 is applied at the input terminal 1 and is sent to the band amplifying circuit 3 in which the chrominance signal S.sub.1 is amplified to a level high enough to carry out the I-axis demodulation. The ACC circuit 5 serves to automatically adjust a gain of the band amplifying circuit 3 for the purpose of keeping the level of the color signal being led to the I-axis demodulating circuit 7 constant.
The amplifying circuit 5 serves to pick up a color burst signal S.sub.2 from the chrominance signal S.sub.1 and amplify the color burst signal S.sub.2. The amplified color burst signal S.sub.2 is input to the 3.58-MHz generating circuit 6 in which a color burst signal is newly produced for the use in the I-axis demodulating circuit 7.
As shown in FIG. 6A, the phase of the I axis is later than the color burst signal S.sub.2 by 57 degrees, so that the phase shift circuit 8 may delay the output of the 3.58-MHz generating circuit by 57 degrees. In the I-axis demodulating circuit, the chroma signal S.sub.1 whose band is amplified and the output of the phase shift circuit 8 are multiplied in an analog multiplier. Then, the resulting I-axis detected output S.sub.3 (see FIG. 6B) appears at the output terminal 9.
The I-axis demodulating circuit 7 shown in FIG. 5 is arranged to have an analog multiplier. On the other hand, the I-axis detecting circuit implemented by a digital circuit has difficulty in detecting an I-axis phase at the sampling point of the chrominance signal S.sub.1 .